In recent years, hybrid cars and electric vehicles have become popular, and more and more vehicles are loaded with batteries in order to obtain electric power. Such a vehicle typically uses an assembled battery including a large number of battery cells connected in series in order to obtain high voltage. The voltages of the battery cells of the assembled battery fluctuate according to use conditions of the vehicle, as is similar to gasoline in gasoline cars. Accordingly, a system for monitoring voltages is necessary to monitor the status of the battery cells.
An example of a voltage monitoring module monitoring voltages of battery cells is already disclosed (Non Patent Literature 1). FIG. 12 is a circuit diagram showing a configuration example of a main part of a voltage monitoring module 300 monitoring voltages of battery cells. Hereinafter, an example that the voltage monitoring module 300 monitors voltages of battery cells EC31 and EC32 connected in series is described. The voltage monitoring module 300 includes switches SW31 to SW33, an A/D converter (ADC) 31, a register 32, a communication circuit 33, a control circuit 34 and input terminals V31 to V33.
A filter circuit 301 and a protection circuit 302 are interposed between the voltage monitoring module 300 and the battery cells EC31 and EC32. The filter circuit 301 includes filter resistors Rf31 to Rf33 and filer capacitors C31 to C33. The high-voltage-side terminal of the battery cell EC31 is coupled to the input terminal V31 through the filter resistor Rf31. The low-voltage-side terminal of the battery cell EC31 and the high-voltage-side terminal of the battery cell EC32 are coupled to the input terminal V32 through the filter resistor Rf32. The low-voltage-side terminal of the battery cell EC32 is coupled to the input terminal V33 through the filter resistor Rf32. The filter capacitors C31 to C33 are coupled between the input terminals V31 to V33 and a ground, respectively. Thus, the filter circuit 301 functions an RC filter, thereby preventing high frequency noise from flowing into the voltage monitoring module 300.
The protection circuit 302 includes protection diodes D31 to D33. The protection diodes D31 to D33 are coupled between the input terminals V31 to V33 and the ground, respectively. Thus, breakdowns of the protection diodes D31 to D33 occur when overvoltage is applied to the input terminals V31 to V33, so that the overvoltage is prevented from being applied to the voltage monitoring module 300.
The switches SW31 to SW33 are coupled between the input terminal V31 to V33 and the ADC 31, respectively. The control circuit 34 controls ON/OFF of the switches SW31 to SW33. The ADC 31 outputs voltage values of the input terminal V31 to V33 to the resister 32, when the switches SW31 to SW33 are turned on. The resister 32 is controlled by the control circuit 34 and outputs the voltage values of the input tettninal V31 to V33 to an external circuit (not shown in the drawings). The external circuit (not shown in the drawings) calculates a voltage of the battery cell EC31 from a voltage difference between the input terminal V31 and the input terminal V32. Similarly, the external circuit (not shown in the drawings) calculates a voltage of the battery cell EC32 from a voltage difference between the input terminal V32 and the input terminal V33. When the voltage of the high-voltage-side terminal of the battery cell EC31 is V+, and the low-voltage-side terminal of the battery cell EC31 is V−, the voltage V_EC31 of the battery cell EC31 can be expressed by the following expression (1).V_EC31=V+−V−  (1)
In addition, a direct current power supply unit for vehicles that has a diagnostic function to diagnose malfunction of a measurement circuit of a terminal voltage of the battery cell is proposed (Patent Literature 1).